The present invention pertains to a stereophonic receiver circuit. More particularly, the present invention pertains to a sterophonic receiver circuit in which switching between stereophonic and monaural modes is carried out automatically in response to the level of the received signal. Yet more specifically, the invention pertains to such a receiver circuit having a reduced noise component in the reproduced output.
A conventional receiver circuit of the same general type to which the invention pertains is shown in the block diagram of FIG. 1. In FIG. 1, an output signal from an IF stage 1 is applied through an electronic volume control circuit 2, a noise canceller circuit 3, and a high-cut control circuit 4 to a stereophonic demodulator cirucit 5, the latter converting the processed IF signal to left-channel and right-channel output signals. The output of the IF stage 1 is also applied to a control voltage generator 6 which produces a so-called "S-meter" signal, the latter being applied to the electronic volume control circuit 2 to set the level of the signal passed to the stereophonic demodulator circuit 5, to the high-cut control circuit 4 for establishing the frequency at which cut-off occurs in the presence of noise, and to the stereophonic demodulator circuit 5 where it controls the operation of a separation control circuit in such a manner that, when noise is present in the received signal above a certain level, stereophonic separation is either defeated or reduced.
FIG. 2 is a graph showing the level of the S-meter voltage in response to changes in the electric field intensity of the signal received at the antenna terminals (not shown) of the receiver. From the zero level to a level S of the received signal, the S-meter voltage changes approximately linearly, while above the level of S, it is substantially constant.
In operation, when the level of the input signal falls below a level M, the stereophonic demodulator circuit 5 switches from the stereophonic receiving mode to the monaural receiving mode, and when the level of the received signal is again above the level S, stereophonic reception is restored.
It would be advantageous to restore the stereophonic receiving mode at a level S' of the received signal intermediate the levels M and S. However, if switching from monaural to stereophonic receiving were effected at the level of S' with the conventional circuit of FIG. 1, the voltage outputted by the control voltage generator 6 would be insufficient, thus requiring a higher-than-desired level of S to be reached before switching back to the stereophonic receiving mode.